1. Field of the Invention
The present invention relates to a small dielectric laminated filter mainly used for a high frequency radio apparatus such as a portable telephone and a communication apparatus.
2. Related Art of the Invention
In recent years, many dielectric laminated filters have been used as high frequency filters for portable telephones. There is, however, a demand for further reduction of the size and thickness of such filters and attention is being paid to planar dielectric laminated filters that can be made thinner than a coaxial type.
An example of the above conventional dielectric laminated filter is described with reference to the drawings.
FIG. 13 shows an exploded perspective view of a conventional dielectric laminated filter. FIG. 14 shows a laminated body constituted by laminating the layers shown in FIG. 13 which are dissembled, as seen from the direction shown by arrow A. FIG. 15 is a cutaway cross sectional view in which the filter is cut along line D-D' shown in FIG. 13.
In FIGS. 13, 14, and 15, reference numerals 101, 102, 103, 104, 105, 106, and 107 designate dielectric sheets. Reference numerals 108a and 108b designate strip line electrodes formed on a dielectric sheet 105. Reference numerals 109a and 109b denote I/O line electrodes, 110a and 110b are notch capacity electrodes, 111 is a coupling line electrode, and these inner electrodes are formed on the dielectric sheets 106, 104, and 102, respectively.
These dielectric sheets are laminated to form a dielectric laminated block on which shield electrodes 115 and 116 are formed on its top and bottom surfaces, respectively. I/O electrodes 117a and 117b and a ground electrode 118 are formed on the outer circumferential side of the dielectric laminated block.
The effects of the dielectric laminated filter configured as described above are described.
In the dielectric laminated filter shown in FIG. 13, the shield electrodes 115 and 116 are grounded via the ground electrode 118. In addition, one end of each of the strip line electrodes 108a and 108b is grounded via the ground electrode 118 to constitute quarter-wavelength strip line resonators. The coupling line electrode 111 and the I/O line electrodes 109a and 109b act as a distributed constant line. A notch capacity is provided between the notch capacity electrode 110a or 110b and the strip line electrode 108a or 108b. The notch capacity electrodes 110a and 110b are connected together via the coupling line electrode 111 to connect the two strip line resonators in parallel via the notch capacity, and one end of the I/O line electrodes 109a and 109b are connected to the notch capacity electrodes 110a and 110b with the other ends connected to the I/O electrodes 117a and 117b in order to constitute a band elimination filter.
To prevent the electromagnetic coupling between the respective electrodes, for example, between the strip line electrodes 108a and 108b, earth electrodes 112, 113, and 114 are formed between the strip line electrodes 108a and 108b, between the I/O line electrodes 109a and 109b, and between the notch capacity electrodes 110a and 110b, respectively.
To prevent the electromagnetic coupling between the strip line electrodes 108a and 108b and the coupling line electrode 111, a shield electrode 120 is formed on the dielectric sheet 103.
A dielectric laminated filter of this configuration is shown in, for example, Japanese Patent Application Laid-Open No. 6-268410.
This design, however, is complicated in this configuration because the electromagnetic coupling between the I/O line 109a or 109b and the strip line 108a or 108b cannot be prevented.
In addition, if dielectric sheets with a large dielectric constant to reduce the size of the filter are used, the electromagnetic coupling between the I/O and the coupling lines and the strip lines is further increased, thereby preventing a good band elimination filter characteristic from being obtained.
Furthermore, the conventional prevention of the electromagnetic coupling between the strip lines 108a and 108b using the earth electrode 112, the electromagnetic coupling between the notch capacity electrodes 110a and 110b using the earth electrode 113, and the electromagnetic coupling between the I/O lines 109a and 109b using the earth electrode 114 is all imperfect and inductance is in fact provided in the earth electrodes 112, 113, and 114. Thus, unwanted electromagnetic coupling occurs between the strip line electrodes 108a and 108b and the earth electrode 112, between the I/O line electrodes 109a and 109b and the earth electrode 113, and between the notch capacity electrodes 110a and 110b and the earth electrode 114.
Furthermore, the earth electrodes 112, 113, and 114 disturb the distribution of electromagnetic fields from the strip line electrodes 108a and 108b, the I/O line electrodes 109a and 109b, and the notch capacity electrodes 110a and 110b to degrade the unloaded Q. As a result, a good band elimination filter characteristic cannot be achieved easily.